Add callback for hot reloading DFNs

[JonasGilg]

Added a callback CosmoScout.callbacks.input.reloadDFNs() to hot reload DFN xml files.

You can easily test it by doing the following:

1. Start CosmoScout VR
2. Move around with "W A S D"
3. Change the keys in the "keyboard_navigation.xml" from "w a s d" to "i j k l"
4. Type CosmoScout.callbacks.input.reloadDFNs() into the JS console.
5. Try moving around with "I J K L", instead of "W A S D"

[coveralls]

Pull Request Test Coverage Report for Build 9741749372

Details

• 0 of 9 (0.0%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 1.146%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/cosmoscout/Application.cpp	0	9	0.0%
<!--	Total:	0	9	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
src/cosmoscout/Application.cpp	3	1.45%
<!--	Total:	3		-->

Totals
Change from base Build 9709642902:	0.0%
Covered Lines:	193
Relevant Lines:	16845

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9741759322

Details

• 0 of 10 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 1.146%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/cosmoscout/Application.cpp	0	10	0.0%
<!--	Total:	0	10	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
src/cosmoscout/Application.cpp	1	1.45%
<!--	Total:	1		-->

Totals
Change from base Build 9709642902:	0.0%
Covered Lines:	193
Relevant Lines:	16845

---

💛 - Coveralls