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.

[igorkorsukov]

@Kavakuo Thank you for your improvement!

A few suggestions:

• I believe that it is better to place the extension removal method in ExtensionInstaller and call it uninstallExtension (because it not only removes, but also causes a reload of extensions, i.e. it is a kind of scenario)
• I think it would be better to rename isUserExtension to canRemoved. So that the name of this flag more accurately reflects its purpose.

[Kavakuo]

@igorkorsukov Thanks for your feedback! I mostly changed the things as you suggested.

I believe that it is better to place the extension removal method in ExtensionInstaller and call it uninstallExtension (because it not only removes, but also causes a reload of extensions, i.e. it is a kind of scenario)

Since only extensions in the new format can be "installed", I first thought that the ExtensionInstaller might not be the best place. However, now I think that's better to have the uninstall function there.

I think it would be better to rename isUserExtension to canRemoved. So that the name of this flag more accurately reflects its purpose.

Instead of canRemoved, I chose isRemovable.