KUrlRequester: fixing handling of start directory

The handling of the start directory in the KUrlRequester was only halfway implemented:
- m_startDir (the private field holding the start directory) is not initialised - the default value of startDir() is an empty URL instead of the current working dir.
- when the start dir changes, it is not always passed to the KUrlCompletion object of the lineedit. The suggestions showed when entering a relative path into the LineEdit might be wrong as a consequence.
- when selecting a file, the start directory does not change to the directory of the selected file - the API doc says it should.
- when the user entered a relative path into the LineEdit, url() returns a relative path instead of an absolute one.

BUG: 92237
REVIEW: 119014
FIXED-IN: 4.14.3