Add First/Last button to Search dialog

The First/Last button finds the first occurrence of the string
in the file if searching forwards, and the last occurrence if
searching backwards.

Note that searching backwards can be done by turning on
"Reverse" checkbox. The currently selected value of
"Reverse" is itself reversed if the First/Last button is Shift
clicked, just like the Search button.

So, if "Reverse" is off, click First/Last for the first match,
shift-click it for last match. If "Reverse" is on, click
First/Last for last match and shift-click it for first match.

Since this behavior is identical to the Search button behavior,
it should be intuitive to the user. If not, they don't need to
use the Shift key and can just use the Reverse checkbox.

src/guiguts/search.py

@@ -136,6 +136,20 @@ def is_valid_regex(new_value: str) -> bool:
             "<Shift-Return>", lambda *args: self.search_clicked(opposite_dir=True)
         )
 
+        # First/Last button - find first/last occurrence in file
+        first_button = ttk.Button(
+            self.top_frame,
+            text="First/Last",
+            takefocus=False,
+            command=lambda *args: self.search_clicked(first_last=True),
+        )
+        first_button.grid(row=2, column=2, padx=PADX, pady=PADY, sticky="NSEW")
+        mouse_bind(
+            first_button,
+            "Shift+1",
+            lambda *args: self.search_clicked(first_last=True, opposite_dir=True),
+        )
+
         # Count & Find All
         self.count_btn = ttk.Button(
             self.top_frame,
@@ -320,11 +334,14 @@ def search_box_set(self, search_string: str) -> None:
         self.search_box.icursor(tk.END)
         self.search_box.focus()
 
-    def search_clicked(self, opposite_dir: bool = False) -> str:
+    def search_clicked(
+        self, opposite_dir: bool = False, first_last: bool = False
+    ) -> str:
         """Search for the string in the search box.
 
         Args:
             opposite_dir: True to search in opposite direction to reverse flag setting
+            first_last: True to begin search at start/end of file
         Returns:
             "break" to avoid calling other callbacks
         """
@@ -335,7 +352,7 @@ def search_clicked(self, opposite_dir: bool = False) -> str:
 
         # "Reverse flag XOR Shift-key" searches backwards
         backwards = preferences.get(PrefKey.SEARCHDIALOG_REVERSE) ^ opposite_dir
-        start_rowcol = get_search_start(backwards)
+        start_rowcol = get_search_start(backwards, first_last=first_last)
         stop_rowcol = maintext().start() if backwards else maintext().end()
         message = ""
         try:
@@ -699,10 +716,10 @@ def _do_find_next(
         sound_bell()
 
 
-def get_search_start(backwards: bool) -> IndexRowCol:
+def get_search_start(backwards: bool, first_last: bool = False) -> IndexRowCol:
     """Find point to start searching from.
 
-    Start from current insert point unless the following are true:
+    Unless "first/last" is forced, start from current insert point unless following are true:
     We are searching forward;
     Current insert point is at start of previously found match;
     Start of previous match is still selected (or it was a zero-length match)
@@ -713,7 +730,11 @@ def get_search_start(backwards: bool) -> IndexRowCol:
 
     Args:
         backwards: True if searching backwards.
+        first_last: True to force searching from beginning/end.
     """
+    if first_last:
+        return maintext().end() if backwards else maintext().start()
+
     start_rowcol = maintext().get_insert_index()
     start_index = start_rowcol.index()
     try: