Fix pressing 'Close' on persistent tabs not working.

[HalfVoxel]

Description

Previously, right clicking on a persistent tab and pressing the Close option would just close the app, but not remove the tab. Clicking Close again on a persistent tab which only had a placeholder would do nothing.

This PR fixes this so that it is possible to close persistent tabs.

Checklist:

• I used indents of four spaces in code and two spaces in documentation
• I have performed a self-review of my own code, it does not generate any errors
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation or will submit them later
• I have recompiled gschgema (if there were any changes)
• Changes to the SASS were done to the variables only or following the proper way

[jntesteves]

If the tab is pinned, the user still needs a way to close the app with the mouse without removing persistence. That's what the Close option does, you can't just remove it without providing a replacement somewhere else, the user needs the option. The Unpin option is there if the user wants to remove that, too.

[PapyElGringo]

I do agree with @jntesteves here too

[HalfVoxel]

That's fair. I agree with that point too.
But I still think that if should close the tab if the tile is just a placeholder. Otherwise to close a pinned tab you first have to unpin it and then click close. Right now the close button on such tabs just silently does nothing. Do you agree?

[PapyElGringo]

I agree that the useless close button on a placeholder is an issue. We could make it Unpin the placeholder and close it or replace it by Open application