Debugger: Improve caching of unnamed structures

Task-number: QTCREATORBUG-9947
Change-Id: I701bb87ecafc4c1260003de51678ab66122aaffa
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>
FakeVim: Operator-pending mappings

Implements :omap (and :onoremap) for defining user mappings for movement
while in operator-pending mode (after commands 'c', 'd', '>' and
others).

Change-Id: I07a1e90bf5c649ef5bb53d1de873817dc7348e19
Reviewed-by: hjk <hjk121@nokiamail.com>