Centralize waiting cursor usage

This PR also fixes a bug introduced by this commit https://github.com/geoadmin/mf-geoadmin3/commit/f5f45bbf429b8771fe85cef1307608ff79ff0608

Step to reproduce were:
1) Open app
2) Type ch (and fast after delete c or ch)

Result: waiting cursor is activated

Expected result: waiting cursor is removed

The counter that was previously implemented in the search directive is now in gaWaitCursor.

Minor: could do `return new waitCursor();` to save a line.