support engine version selector

ThorVG is planning to upgrade its engine version (v0 -> v1). If it supports both major versions, it should provide an option to select the engine via a dropdown control, for example:
[v1, v0] (with v1 selected by default).

This will allow users to test and verify both versions using a single viewer tool.

for now, we can provide theses:
[v1.0-preX, v0.15.x]