Set of fixes in markers handling code

- Fixed a bug related to not working marker deletion in certain conditions

- Fixed a bug with new marker creation: after having removed a marker, if the
  user invoked the creation of a new one, that was invisible (sometimes) and
  the description contained random text from pre-loaded markers

Basically, these bugs (which existed for a while, as it seems) are related to
wrong assigning of markerId-s and not clearing local flags/variables which are
shared between marker management related function calls.
Merge branch 'markers_fix'
Fixed a concept bug introduced in previous commit

The use of max id is right, but the value should be determined for the whole
markerModel, not just a subset of markers for given face.
Merge branch 'master' of github.com:develersrl/demo-qml