Per-command monitor handling should have a "screen" prefix

[ThomasAdam]

There are some commands which have a literal screen prefix to denote which screen/monitor the command applies to, such as:

EWMHBaseStruts screen HDMI-1 0 0 0 0

But there are many others which do not, such as:

GotoDesk HDMI-1 0 0

This inconsistency makes command parsing a nightmare, since the overall length is variable; the presence of a literal screen keyword to denote the presence of a given screen would help solve this.

Commands which need changing:

• GotoPage
• GotoDesk
• GotoDeskAndPage
• Scroll

Don't forget to update hard-coded instances of these in modules:

• FvwmButtons
• FvwmPager