[filter-control] Always return the same control container target

[jayaddison]

🤔Type of Request

• [x] Bug fix
• [ ] New feature
• [ ] Improvement
• [ ] Documentation
• [ ] Other

🔗Resolves an issue?

Fix #7171.

📝Changelog In the filter-control getControlContainer function, always return the same target control container, regardless of whether the height (fixed-header) option is enabled, and regardless of the filter control _initialized state when it is.

• [ ] Core
• [x] Extensions

💡Example(s)?

• Before - https://live.bootstrap-table.com/code/jayaddison/18021
• After - https://live.bootstrap-table.com/code/jayaddison/18022

☑️Self Check before Merge

⚠️ Please check all items below before reviewing. ⚠️

• [ ] Doc is updated/provided or not needed
• [ ] Demo is updated/provided or not needed
• [ ] Changelog is provided or not needed

Edit: untick the reviewer checkboxes that I'd filled by mistake.

[jayaddison]

@wenzhixin I haven't yet heard from any affected bugreporters, but I believe that this pull request solves the problem described in #7171 when the filter-control extension is used in combination with the mobile extension. I'd be glad for any code review and suggestions.

[jayaddison]

Due to my unfamiliarity with this part of the bootstrap-table codebase, combined with an apparent absence of demand (and therefore assistance testing the fix), and also the fact that this feature isn't currently required by the project I'm working on (it would be nice, because it might allow removal of other dependencies) -- on balance I'm going to close this to reduce my personal tasklist.

To anyone who might find this could useful: please feel free to retrieve the commits from this branch and continue work from here. Using git fetch <remote> pull/7445/head:localbranch should allow retrieving up to commit dda21c397752e7b10b77f5f18026ab55799811a9 into a local branch.