Fix: Detect changes correctly in conandata and conanfile

[czoido]

Some more fixes:

• Correct logic for overwritting guarded files
• Don't crash when reading conandata if the file is modified with unknown keys
• Remove the guard comment, and it shows empty lists.
• Remove a requirement from the conandata and click the "list only installed" button; it still shows the removed one.
• Remove the guard comment, click the add requirement button on any reference, and the pop-up window shows that it has been added, but in reality, it hasn't (this last part is expected; what you don't expect is the pop-up window).

Closes: #242