Refactor 'immediate search' feature

Currently, code just checks whether a text-edited button uses a given icon (VIEWZOOM) to decide to apply changes on each typed char.

This patch adds a propper button flag (UI_BUT_TEXTEDIT_UPDATE) and a dedicated RNA flag (PROP_TEXTEDIT_UPDATE) for that.
It's also now usable not only for text buttons, but also for example for num buttons when in 'text edit' mode, etc.

It also fixes an actual bug, which is for text properties, in 'immediate' mode, hitting escape would not restore org value, because `ui_apply_but_TEX()` would set its orgstr to NULL on first call (giving it to `but->rename_orig` instead of copying it).

Note no change in behavior is expected from user POV.
Update for addons using that 'VIEWZOOM' icon 'feature' will follow (if any).

Reviewers: campbellbarton

Reviewed By: campbellbarton

Projects: #user_interface, #bf_blender:_next

Differential Revision: https://developer.blender.org/D938

source/blender/python/intern/bpy_props.c

@@ -64,6 +64,7 @@ static EnumPropertyItem property_flag_items[] = {
 	{PROP_ANIMATABLE, "ANIMATABLE", 0, "Animatable", ""},
 	{PROP_LIB_EXCEPTION, "LIBRARY_EDITABLE", 0, "Library Editable", ""},
 	{PROP_PROPORTIONAL, "PROPORTIONAL", 0, "Adjust values proportionally to eachother", ""},
+	{PROP_TEXTEDIT_UPDATE, "TEXTEDIT_UPDATE", 0, "Update on every keystroke in textedit 'mode'", ""},
 	{0, NULL, 0, NULL, NULL}};
 
 #define BPY_PROPDEF_OPTIONS_DOC \