Add support for importing loadouts from build XML

[deathbeam]

(I totally did not made this PR just because I was too lazy to merge my 2 pobs together manually :d)

Adds support for importing only loadouts (item sets, skill sets, tree specs, config sets) from a build XML to current build without overwriting anything else.

Closes #7843

Example pob 1: https://pobb.in/4E0oETFkyZi0

Example pob 2: https://pobb.in/xGLaSDnPOuKw

Example merged pob: https://pobb.in/wb54Pkh83oR9

Merging process:

Open pob, go to import, add xml or link, select import loadouts only:

After import press:

[deathbeam]

Added also prefix + warning window:

[deathbeam]

Added support for importing config sets as well so now all sets should be imported properly

[indivisible]

Very nice feature

Some thoughts:

1. since it appends everything else, it could also append notes
2. it might be cleaner if you generate an export XML tree and then modify it to add the pieces from the imported data. This could avoid adding changes to all the existing loader functions. The drawback of this would be some duplicated functionality in the codebase
3. animated minion itemset references are not corrected. These are referenced skillMinionItemSet attribute of Gem tags

[deathbeam]

Thanks for the review.

1. It do not appends everything else it appends specifically loadouts. Notes are not part of the loadouts so im not sure if its good idea to combine them, could create a mess, but should not be hard to add either way
2. I wanted to avoid this. I think modifying the loader functions to simply not do reset is correct, but the item id mapping I was unsure if I should do inside or outside. I can def move the remapping part from inside of tree tab at least, will be probably cleaner, but I did not wanted to duplicate a lot of code (originally I had the code duplicated but it felt very brittle to do that just for this)
3. Hmm good catch, I will try to fix this

[deathbeam]

Ok moved the tree id remapping to loadouts import and also added item set remapping