ide: External mode

[mirek-fidler]

This is special mode for assembly:

This is designed to use theide capabilities in non-U++ projects that cannot use U++ packages / build system.

In this mode, package files are actually stored in theide configuration (in subfolder with that same name as assembly). All subfolders of "nest" are considered package (in package selector) if they contain files with prescribed masks. Also, when adding a package to the project and the package is yet empty (usually because its package file does not exist yet), it automatically adds all files from the "package folder".

The ultimate aim is to have Assist++ working for e.g. visual studio or cmake projects, with "light emulation" of U++ package system.