Requires Qt 5.15

We migrated to the new `Connections` syntax and we want to use the new
input handlers, that were improved in recent releases.

Qt 5.15 is now available in most Linux distributions and it's the last
version before the 6.x series.

CMakeLists.txt

@@ -16,7 +16,7 @@ endif()
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake;${CMAKE_CURRENT_SOURCE_DIR}/cmake/3rdparty")
 
 ## Set minimum versions required:
-set(QT_MIN_VERSION "5.12.0")
+set(QT_MIN_VERSION "5.15.0")
 
 ## Liri specific setup common for all modules:
 include(LiriSetup)

README.md

@@ -17,7 +17,7 @@ Compiler requirements:
 * [gcc >= 4.8](https://gcc.gnu.org/gcc-4.8/) or
 * [Clang](http://clang.llvm.org/)
 
-Qt >= 5.12.0 with at least the following modules is required:
+Qt >= 5.15.0 with at least the following modules is required:
 
  * [qtbase](http://code.qt.io/cgit/qt/qtbase.git)
  * [qtdeclarative](http://code.qt.io/cgit/qt/qtdeclarative.git)