Extensions: Added uninstall and update process

[Kavakuo]

Hi all,
I recently played around with the new extension format (.mext) and noticed that it is currently not possible to ship updates for an extension without performing a manual update process (deleting the folder, drag&drop the extension for installation). I didn't find any issues for this, so I thought this could be a small fun weekend project. So this PR introduces two new features improve the process:

• Added two new modal popups (1e981f7)
  • When the user tries to install the extension with the same version
  • When the user tries to install the extension with another version. The popup contains the current and old version number. If the user agrees, the old version is removed and the new version is installed.
• Added a button to the extension panel, to be able to remove/uninstall an extension (e371e3d)

I am no C++ expert, so please let me know if you see any room for improvements that I should change.

Remaining ToDos:

• Localization of the strings in the modal popup. I probably need some help here, since I don't know the workflow.

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[Kavakuo]

Thanks for the fast review @cbjeukendrup! I added a commit to address your suggestions.