Rework 'file name widget' popover and dialogs -- alternate route

Alternative to !1308 (closed), cherry-picking major parts from there.

This alternative approach tries to:

• preserve current behavior regarding delayed duplicate name feedback (which avoids interruption while still typing more characters)
• preserve git changes context (keep git blame useful)
• replace inheritance with delegation
• embrace reactive programming with signals, bindings and expressions in the UI declaration

24 files changed, 1727 insertions(+), 2261 deletions(-)