Add methods to scroll command line and detect EOF.

Working on issue #218.

Will be used to implement MORE mode when printing messages to command
line.
Rename Command line MORE mode to MESSAGE

Actual "MORE" logic will be implemented in vrapper core, not the UI.

For issue #218.
Add Message mode which displays the MORE prompt.

Necessary to support issue #208.

To print messages, extend the AbstractMessagesCommand which will do all
the mode switching.
Don't reset command line UI mode after mode change

Because setMode(DEFAULT) would happen after a ChangeModeCommand, the
command line UI would return to DEFAULT mode after a *MessagesCommand
changed it to MESSAGE.

Shuffle code around, making sure to set the DEFAULT command line UI mode
in all other cases.

Necessary for issue #208.
Insert command line UI REGISTER marker just once.

Repeatedly setting the command line UI mode to REGISTER would insert
several double-quote marks.
Implement :tabs command.

This command lists all open tabs with their numbers and the file
displayed in them.

Only editors using a FileEditorInput are currently useful, all other
editors will only print a short display name, which depends on the
editor.

Work on #208.
Mark current editor in tablist.

Put a ">" marker in front of the current file.

Fixed #208 as the feature seems complete.