Add icons for some menu options in KMail

Summary:
In KMail, the following menu options did not have icons:

- Settings -> Add Accounts (Main window)
- Settings -> Configure Filters (Main window)
- Go -> Jump to Folder (Main window)
- Tools -> Spellchecker (Compose window)

This commit adds relevant icons for these.

Reviewers: #kde_pim_kmail, dvratil

Reviewed By: dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D28063