clien: pdf-gen: load logo externally; add prepare step * Change the way how the logo to be placed in the pdf is loaded: It can be configured in the `.env.local´ file, otherwise a placeholder will be loaded from the static assets. The placeholder has not been placed in `assets` because it won't be needed if a real logo has been specified and thus should not compiled in by default. * Add a mechanism to do asynchronous preparations before rendering. This is needed for the completeion of the Image() loading, but it also shows how asynchronous preparational step can be added in principle. * Disable the start button when the current rendering is in progress.

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136aaa7f00af0b500c5c75f9fd9aabfc7fa526ee v1.1.0
724758455a4e44d25b096f00bcb353ceda0e0d57 v2-preview20190111
0059aa870a396e2112efae43f822c0c133454d7a v2