Improve versioning of target Godot version to support a range

[akien-mga]

Currently when you submit a new asset, you had to select the target Godot version from:

This is insufficient because:

• You don't know if selecting 3.0 means that it will work with 3.0.4 (AFAIK it does, but it's not clear)
• You don't know if selecting 3.0 means that it won't be available on 3.1 (AFAIK it's still available on 3.1, as I still see assets in my build from the master branch, but it's not clear either)
• If you want to target the next stable version, you have to wait for it to be added (#155)

Ideally there should be a way to specify that an asset has a minimum supported version (e.g. 3.0.2), and a maximum supported branch (e.g. 3.0.x, or 3.x - while keeping the >= 3.0.2 requirement). As well as an option for "any version above min requirement", if there aren't any known compatibility breakages in current Godot version (with possibility in a later edit to restrict this max to e.g. 3.x if it happens that 4.0 is not compatible).

Ref #100.

[Zylann]

Another problem I foresee is that with the incoming Godot 3.1, I will have to upgrade one of my plugins to exploit features only found in 3.1... which means I either have to painstakenly multiply code paths in the plugin to support both versions (which may not always be possible since it may involve complete rewrite of some areas), or I have to upload the addon a second time with some [3.1] and [3.0] prefixes in the name?

[bojidar-bg]

@Zylann I think #143 will help with that.