Prepare release 5.1.0

As preparation for the upcoming 5.1.0 release the following todos have to be taken care of:

• Merge the existing 5.0 branch into master. After this step, no more changes shall be pushed to the 5.0 branch
• Create new 5.1 stable branch off of master
• Update i18n repos to use 5.1 branch as new source for stable
• Announce availability of the 5.1 branch on the developer and kde-i18n-doc mailing list
• Announce string freeze by mail to developer and i18n-doc list
• Add version information to org.kde.kmymoney.appdata.xml (see https://phabricator.kde.org/T12566)
• Announce code freeze by mail to developer list
• Alter all bugs which show Fixed in 5.0.9 to show Fixed in 5.1.0 instead
• Prepare release notes to be published on web-site
• Prepare ChangeLog to be published on web-site
• Fix the VS2019/QT5.15 KChart build blocker #15 (closed) Windows