[ FEATURE REQUEST ] MERGE MULTIPLE FILES AT ONCE

files can only be merged one at a time - this is painful when importing resused subassemblies or submodels for complex model variations.

Add capacity for multiple selections and merge selected files in order of selection/alphabetical or add option toggle in merge window dialog to activate multiple selections as safety.

**Describe alternatives you've considered**
n/a

**Additional context**
n/a
