enhance: support tools installed via flatpak

[NathanBaulch]

This PR is a bit experimental.
I have various tools installed via Flatpak (Meld, VSCodium, etc) that I'd like sourcegit to integrate with. Currently all diff and merge tools must resolve to a known binary file, whereas Flatpak apps are launched via the flatpak binary (eg flatpak run org.gnome.meld). I've figured out some workarounds to get ExternalTool and ExternalMerger working but I'm reluctant to make any significant changes because I can only test on Linux.

[love-linger]

I suggest directly creating a command-line tool for Flatpak applications, so it can be used as normal application. For example:

1. Create /usr/bin/meld，and fill it with following contents：

#!/bin/bash

/usr/bin/flatpak run org.gnome.meld $@

2. Change the file mode with chmod u+x /usr/bin/meld

[NathanBaulch]

Digging deeper, it looks like Flatpak already does this for me! Specifying the following install path works for my per-user Meld installation: /home/nathan/.local/share/flatpak/exports/bin/org.gnome.meld 👍
It would be cool if sourcegit could figure this out automatically, but at least I have something workable.
It would also be cool if it could automatically detect these in ExternalTool, which is not something I can override in preferences.

[NathanBaulch]

I've updated this PR with much cleaner Flatpak external tool support based on inspecting the system-wide and per-user flatpak/exports/bin folders. Changes are limited to Linux.cs and ExternalMerger is no longer touched.

If sourcegit can have special support for JetBrains Toolbox then it seems reasonable that it can also support Flatpak on Linux.