Fix Chromium web app grouping in Cinnamon with manual StartupWMClass detection

[sohailb108]

In Cinnamon, different Chromium web apps can get the icon of the first one launched and group together.

I found that StartupWMClass in the .desktop file didn't match the window class from xprop WM_CLASS and updating StartupWMClass to match fixed the issue.

Since Chromium sets the window class at runtime, correct automatic detection at add time sounds impossible.
So this PR adds a manual fix in Web Apps with a new toolbar button.

How to use it:

• Select a Chromium-based web app from the list.
• Click the newly added button (rightmost one) to launch it and start detection.
• The cursor changes to a cross.
• Click the launched app window.
• xprop WM_CLASS is read, and the first WM_CLASS value is saved into StartupWMClass in the corresponding .desktop file.

How to test:

• Create two different Chromium web apps.
• Launch both and confirm they are grouped together with the same icon in the panel.
• Apply the fix for each app using the new button.
• Launch both again and confirm each one appears separately with the correct icon.
• You can also open the .desktop file and verify that StartupWMClass was updated to the WM_CLASS value returned by xprop.

Changes:

• Added a new toolbar button (rightmost) to the Web App management window.
• Implemented the detection and update routine.

Fixes #100