Installer: Improved alert text, consistent use of "Haiku User Guide"

Point specifically to the SoftwareUpdater topic of the Haiku User Guide
to make the hint more useful.

Always use consistent spelling: "Haiku User Guide"

Translated the German User Guide package info.

Change-Id: I8f3b2a5a3f27caf9b5eea41a1a7a86c2e91e668a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3412
Reviewed-by: Niels Sascha Reedijk <niels.reedijk@gmail.com>

Fix 'EULA' to mention the Quick Tour instead of Welcome page

Fixes #16108

Change-Id: Ib19b8950f6ed946444a94581f578af39f4e04bf4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2787
Reviewed-by: Emir Sarı <bitigchi@me.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Installer: update 'EULA' window

* De-escalate the severety of data loss from "high risk" to "risk".

* Remove the advice to not create partitions with DriveSetup. It's been tested
and been working well since the last alpha. Do recommend making backups of
all partitions.

* Remove description how to add Haiku to Grub. It's too much detail and the
user would have to memorize the info. Add a link to our Booting guide
instead.

* Mention BootManager as an alternative to boot Haiku, refering to the
Userguide for more info.

* Alpha -> Beta

* Only give normal thanks instead of "a lot" and don't beg to like us.
Sounded a bit desperate...

Change-Id: I212513fde01287f5d1c9c5c9891827aaf0b7e1ea
Reviewed-on: https://review.haiku-os.org/502
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
(cherry picked from commit 1edc131debcf8ea1c9cd9eafd41c356f0432d9a5)
Reviewed-on: https://review.haiku-os.org/508

Use B_USE_WINDOW_SPACING

Use B_USE_WINDOW_SPACING as inset from contents to window border.
Some whitespace cleanup.

Removed GRUB 1.x hints from Installer's 'EULA' window

Pretty much everyone should be using GRUB 2 by now.

Installer: closing EULA with Alt-W continues to main app

* Shortcut issue - closing EULA window with Alt-W continues to main
application window instead of sticking windowless application in
the Deskbar tasks list. The idea is closing only the one window
of two;
* Fixes #9982.

- GCI 2013

Removed GRUB 1.x hints from Installer's 'EULA' window

Pretty much everyone should be using GRUB 2 by now.

Installer: closing EULA with Alt-W continues to main app

* Shortcut issue - closing EULA window with Alt-W continues to main
application window instead of sticking windowless application in
the Deskbar tasks list. The idea is closing only the one window
of two;
* Fixes #9982.

- GCI 2013