Add DB_DIR placeholder for autoopen urls

[stingray21]

Type of change

• ✅ New feature (non-breaking change which adds functionality)

Description and Context

Add placeholder {DB_DIR} for the url of the autoopen functionality

Adding placeholder for the url of the autoopen functionality would it make easier to sync a database with a child database on multiple devices via file sharing services.

e.g. {DB_DIR}/passwords.kdbx

Currently relative paths and absolute paths work in KeepassXC (#1053).
But relative paths do not work in Keepass2 or other clients i.e Keepass2Android, whereas path placeholders work in those clients.

Keepass placeholders:
https://keepass.info/help/base/placeholders.html

Fixes #3863

Testing strategy

created databases linked them to the parent database, using different url styles (relative path, file:///, and with {DB_DIR})

built it with following settings:
cmake -DWITH_TESTS=ON -DWITH_ASAN=ON -DCMAKE_BUILD_TYPE=Release ..

I ran the make test and these tests failed (but I don't know what to do about it):
1 - testgroup (Failed)
15 - testopvaultreader (Failed)

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]
  see testing section
• ✅ I have compiled and verified my code with -DWITH_ASAN=ON. [REQUIRED]

[stingray21]

I know that there are still some parts that definitely need to be changed. I tried to understand how the code works and to make the {DB_DIR} placeholder work to best of my knowledge (I'm still pretty new to C++ and this project).

• I didn't understand what the if statement in the switch cases is supposed to check (e.g. if (placeholderType(url()) == PlaceholderType::Url)). Therefore I put a proxy statement that is always true.
• I know that I am not supposed to declare variables within the switch statement, but I am unsure where to declare it. I thought it only needs to be declared in the DB_DIR case.
• I changed entry->url() to entry->displayUrl(), otherwise the path would be replaced in the gui but the child databases do not get opened
• not all the tests succeeded, but I do not know what I should do about that or how to resolve it

[stingray21]

I still have the useless if (PlaceholderType::DbDir == PlaceholderType::DbDir) statement in the file Entry.ccp on line 899. What am I supposed to check here?

Also, where can I learn more about the tests? To figure out why test 1 and 15 failed.

[droidmonkey]

Sorry for the extreme delay merging this, its good to go now.