Cleanup config initialization, add local config options

* Fix #5313, allow specifying local config path using environment variable and command line flag
* Add command line flag `--localconfig <path>` to specify a file path to use for the local configuration settings.
* Add environment variable support to set config files paths: `KPXC_CONFIG` and `KPXC_CONFIG_LOCAL` to override default locations.
* Reorder startup sequence to load specified config files earlier to allow for theme settings and other early options to be picked up.
* Removed old command line option `--pw`, no longer used.

* Attempt a fix of application not closing when last window is gone. Only set `QApplication::setQuitOnLastWindowClosed(true)` when tray icon is enabled instead of always.

docs/topics/UserInterface.adoc

@@ -48,4 +48,41 @@ image::compact_mode_comparison.png[]
 
 === Keyboard Shortcuts
 include::KeyboardShortcuts.adoc[tag=content, leveloffset=+1]
+
+// tag::advanced[]
+=== Command-Line Options
+You can use the following command line options to tailor the application to your preferences:
+
+----
+Usage: keepassxc.exe [options] [filename(s)]
+KeePassXC - cross-platform password manager
+
+Options:
+  -?, -h, --help               Displays help on commandline options.
+  --help-all                   Displays help including Qt specific options.
+  -v, --version                Displays version information.
+  --config <config>            path to a custom config file
+  --localconfig <localconfig>  path to a custom local config file
+  --keyfile <keyfile>          key file of the database
+  --pw-stdin                   read password of the database from stdin
+  --debug-info                 Displays debugging information.
+
+Arguments:
+  filename(s)                  filenames of the password databases to open (*.kdbx)
+----
+
+Additionally, the following environment variables may be useful when running the application:
+
+[grid=rows, frame=none, width=75%]
+|===
+|Env Var                        | Description
+
+|KPXC_CONFIG                    | Override default path to roaming configuration file
+|KPXC_CONFIG_LOCAL              | Override default path to local configuration file
+|SSH_AUTH_SOCKET                | Path of the unix file socket that the agent uses for communication with other processes (SSH Agent)
+|QT_SCALE_FACTOR [numeric]      | Defines a global scale factor for the whole application, including point-sized fonts.
+|QT_SCREEN_SCALE_FACTORS [list] | Specifies scale factors for each screen. See https://doc.qt.io/qt-5/highdpi.html#high-dpi-support-in-qt
+|QT_SCALE_FACTOR_ROUNDING_POLICY | Control device pixel ratio rounding to the nearest integer. See https://doc.qt.io/qt-5/highdpi.html#high-dpi-support-in-qt
+|===
+// end::advanced[]
 // end::content[]