Fix displaying a custom proposal set in AY

[mvidner]

• https://trello.com/c/Bnc4V82U
• https://bugzilla.suse.com/show_bug.cgi?id=1190294 (L3Q)

Filter the installation proposals (in the Installation Settings screen) according to the AutoYaST profile even before tab switching

The filtering code was only used after a tab change (in switch_to_tab), here apply it also for the initial display.

Manual Test

I tested the fix with a profile that limited the proposals to software and language:

<profile xmlns="http://www.suse.com/1.0/yast2ns"
  xmlns:config="http://www.suse.com/1.0/configns">
  <general>
    <mode>
      <confirm config:type="boolean">true</confirm>
    </mode>
    <proposals config:type="list">
      <proposal>software_proposal</proposal>
      <proposal>language_proposal</proposal>
    </proposals>
  </general>
...

(full profile at https://gist.github.com/mvidner/909181a348483fd367ab2c689880a293#file-minimal-sle-15-proposalmodulebug-sw-lang-xml)

Screenshots of a Tabbed Proposal, Before and After

This test used a stock control.xml file which has Installation Settings show two tabs, Overview and Expert.

Initial display	Switched to 2nd tab	Switched back

Screenshots of a Non-Tabbed Proposal, Before and After

This test used a modified control.xml that limited the proposal set to software, timezone, language, presenting them without tabs.

• control.xml.diff
• the full new control.xml

Before	After

[coveralls]

Coverage decreased (-0.01%) to 40.02% when pulling e465dee on ay-custom-proposal into d745ba6 on master.

[dgdavid]

Although looks like it is not the final patch for https://bugzilla.suse.com/show_bug.cgi?id=1190294 I think you can go ahead and merge this patch. LGTM, thanks!