The chooser allows choosing variables from a list, and inserting them
into previously focused line edit, text edit, or plain text edit.
Variables are now registered with a description in the variable manager.
Also make the QT_INSTALL_BINS --> CurrentProject:QT_INSTALL_BINS for
consistency.

It's done via a external tool description now.

This overrides ordering of *all* tools. Which is suboptimal.

Reordering isn't persistently saved yet.

Because otherwise implementing drag&drop is impossible.

Adding new categories is not there yet though, neither is moving tools
through the categories.

Not possible to reset them yet.
Saves the change in the settings instead of writing new tools xml files.
This probably needs to be adapted for tools that are "local" from the
beginning.

No editing possible yet.

It now follows quoting rules.

Take it as an expected file change.

External 'sort' now works as it should.

* Handle <input> tag.
* Pass input to running process.
* Search for executable in path.
* Add "CurrentSelection" variable.

Remaining issue: ReplaceSelection output handling.

Most of the output handling, error handling etc is missing atm.

Not handled yet.

For now that removes the CURRENT_DOCUMENT variable and derivates.