How to reproduce:
1. Open the attached "case_1148709-EmitSignalNoSave" project
2. Open the "New Timeline" in Timeline Window
3. Select the Signal Emitter marker and change the Emit Signal property from "Signal1" to "Signal2"
4. Close and reopen the project, then inspect the Signal Emitter again

Expected results: The Emit Signal property is set to "Signal2"
Actual results: The Timeline asset did not get marked as dirty, therefore the Emit Signal property did not get saved and is still set to "Sginal1"

Reproducible with: 2019.1.0f2, 2019.2.0a13

Note:
1. Could not test with 2019.1.0a10 and below as Timeline Signals were introduced in 2019.1.0a11
2. Changing anything else in the Timeline (f.e. adding a Track or another Signal Emitter) causes the changes in "Emit Signal" properties to be saved