Add completion for PRO-file tags in editor

Add PRO-file's editor ability to drop down a list of suggested values if this list is limited - it is a codecompletion like feature.
Another way is to show in context menu additional items relative to the cursor position (for example, `Add Linker Search Path...`, `Add link library...`, `Add Include path...` and so on) - it is a context menu feature.
For file system items (i.e. paths to directotories or files) editor can supply item `Browse...` in context menu to allow user select file or directory from native standard dialog.
It will be great to have $$... like variables (they can be defined in PRO-file) support in these dialogs, i.e. dialog can get absolute, relative to project's root directory or variables based paths (if you select variable in the dialog's variables list, which is loaded on dialog show event, for example).

These my feature requests will be very useful for newcomers as much as for professionals - you don't need to search for qmake help to edit PRO-file.

With regards, Qt user !