Improve QgsSearchCombo

Changes:
- [Feature] replace reset button with ExtJS Trigger to clear search
- position cursor in search field after clearing
- display clear button only if anything is there to be cleared
- ignore user pressing enter during search
- collapse result list when search string is less than minChars
- ensure use of reverseAxisOrder for boundingBox of selected search
  result
Merge pull request #39 from bstroebl/clearSearchCombo2

Improve QgsSearchCombo