Implement support for X11 selection clipboard

Reformat SWTClipBoardRegister, allow to pass clipboard type.

On platforms other than Linux, the regular clipboard will be bound to "*
just like it was before.

Work for #551
Add support for string-set config options.

Used to more properly support the 'clipboard' setting, it is actually a
composite configuration option.
Rework 'clipboard' as a string-set option.

Also add 'unnamedplus', 'autoselectplus', 'exclude:' as possible values
for the 'clipboard' option.
Add support for 'clipboard=unnamedplus'

Since the Linux version now supports the selection clipboard, the
"unnamed" variant of the 'clipboard' option *will* copy to the selection
clipboard whereas it used to copy to the system clipboard.

Vim on the other hand always distinguished the clipboards by having an
"unnamedplus" setting, so this commit adds this option to be compatible
with Vim.

Users who currently have `set clipboard=unnamed` are thus advised to
change it to `set clipboard=unnamedplus`.

Windows or Mac users should see no change though.
Simplify selection initialization.

The distinction between inclusive/exclusive is handled in
VisualMotionCommand, also align the other visual modes.
Implement autoselect(plus) like in Vim.

Vrapper used to set 'clipboard' to "autoselect" but it wouldn't properly
handle it. Vim actually copies the current visual selection to the
selection or system clipboard when this option is set.

Fixes #551.