[ModManager] Add column for applied state and filter applied/not applied mods

[Skylark13]

Hello,

Long-time user of mod managers for Elder Scrolls games (Morrowind, Oblivion and Skyrim) and Dragon Age games too.
I recently started using FMM because I wanted to use some mods in a playthrough of Mass Effect Andromeda.
I found it hard to figure out which mods were currently applied in a large list of mods. For example:

• if I had a few different variants of a mod, and only one had to be enabled at a time,
• or if I wanted to test out a mod but wasn't sure I wanted to use it in the end, so I installed it but might not keep it applied or might want to apply it, test, unapply it, test again, etc.

So I added a column in the "Available Mods" table where there will be a checkmark if the mod is applied.
Also I added toggle buttons to show only applied or not applied mods in combination with the filtering by name.
Finally, I added a status bar at the bottom of the table to show how many mods are either available (if not filtering) or pass the current filter (if filtering).

Bear in mind that I am not that experienced in C#. If there are ways to do something better, let me know. In particular I had trouble applying the right Binding with the IValueConverter for the checkmark to show up next to each available mod in the table. The converter needs access to the list of applied mods, and I didn't know how to do that in the XAML, so I went down the path of creating the DataTemplate for that column in code, which took a while to find something that worked. The rest is pretty straightforward but let me know if you prefer I do something in some other way.

Also if you prefer some other images for "applied" and "not applied" state, let me know. I chose a checkmark in a circle, and an X in a circle, respectively, and I tried to follow the style of the rest of the icons.

Here are some gifs to show the additions in action: