Revert "[Core] Just keep the "Add tab" button on the left no matter what."

This reverts commit a9063515e9dcfabaf169f1880e9f7d5a3b0dbc4a.

Conflicts:
	src/core/separatetabbar.cpp
	src/core/separatetabwidget.cpp
[Core] Added GetAntiCloseButtonPosition().
[Core] Last tab is always a plus button.

Removed all mentions of LastTab_.
[Core] Add tab button is always hidden.
[Core] Proper button on the last tab.
Merge branch 'master' of github.com:0xd34df00d/leechcraft