Show external scripts in tabs

- External scripts (included into the main script using include())
use the same tab class as the main script tab, ie. ScriptTab
- Can be opened from the projects dock (context menu of the script item)
- Gets opened automatically when interrupting in external script code
- Each project can open a list of included external scripts in tabs
- Move Project::executionLine to ScriptTab, ie. each script tab gets
it's own execution line, move code for marking the execution line
to ScriptTab::setExecutionLine()
- Add ShowExternalScript project action, can be used with the file path
to the file to open as data or without data to show a file dialog
- Fix const for AbstractTab::fileName()
and AbstractDocumentTab::fileName()