After fixing superficial issues go and work over Client's fullscreen control.
This way we:

• check first for what the rule wants uis to do,
• do only proceed if there is a change from/to fullscreen
• remove code, that becomes unneeded by this.

This goes with the assumption, that the current fullscreen state is always
correctly stored in the m_fullscreenMode variable, but the previous code
implicitly did the same at numerous occasions, just not in a consistent
manner.

Manually and auto tests still pass.